Process For The Preparation Of Multimetallic Catalysts That Can Be Used In Reactions For Transformation Of Hydrocarbons

ABSTRACT

The invention relates to a process for the preparation of a catalyst comprising:
     a) The preparation of a colloidal oxide suspension of a first metal M1 that consists in the neutralization of a basic solution by an acidic mineral solution that contains the precursor of the metal M1,   b) Bringing into contact the precursor of the promoter M2, either directly in its crystallized form or after dissolution in aqueous phase, with the colloidal suspension that is obtained in stage a),   c) Bringing into contact the colloidal suspension that is obtained in stage b) with the substrate,   d) Drying at a temperature of between 30° C. and 200° C., under a flow of air.   

     The invention also relates to a process for the treatment of an olefinic fraction that uses the catalyst prepared said preparation process.

This application is a continuation of International ApplicationPCT/FR06/00326 filed Feb. 10, 2006, which claims benefit of priorityfrom French Application 05/01.967 filed Feb. 25, 2005.

The processes for conversion of hydrocarbons, such as vaporeforming orcatalytic cracking, are performed at high temperature and produce alarge variety of unsaturated molecules, such as ethylene, propylene,linear butenes, isobutene and pentenes. By the same token,polyunsaturated compounds that contain the same carbon number are alsoformed: acetylene, propadiene and methyl acetylene (or propyne), 1-2 and1-3 butadiene, vinyl acetylene and ethyl acetylene, and finally othercompounds with a boiling point that corresponds to the gasoline fraction(C5+). All of these compounds should be eliminated to make possible theuse of these different fractions in the petrochemistry processes such asthe polymerization units. Thus, the C2 steam-cracking fraction has thefollowing mean volumetric composition: 1.2% by weight of acetylene,83.5% by weight of ethylene and 15.3% by weight of ethane. For the C3fraction, the same type of distribution with a predominance of propylene(90% by weight) and contents of propadiene and methyl acetylene on theorder of 3 to 8% by weight are found. The specifications that relate tothe concentrations of these polyunsaturated compounds for thepetrochemistry and polymerization units are very low: 20-30 ppm of MAPD(methyl acetylene and propadiene) for the propylene chemical quality andless than 10 ppm, and even up to 1 ppm, for the “polymerization”quality. The elimination of the MAPD can be done by distillation or byextraction by solvent.

The selective hydrogenation is gradually imposed because this processmakes it possible to obtain propylene yields, in the case of the C3fraction, that are greater than the above-mentioned processes(102-103%). This reaction makes possible the conversion of the methylacetylene and the propadiene into propylene by avoiding the totalsaturation that leads to propane. It can also be carried out in gas orliquid phase with a preference for the liquid phase that makes itpossible to lower the energy cost and to increase the cycle time of thecatalysts. The operating conditions that are usually applied are apressure of about 2 MPa, a temperature of between 10 and 50° C., and ahydrogen/(hydrocarbon to be hydrogenated) molar ratio of between 1 and2. The selective hydrogenation reaction can also be carried out in agaseous phase; in this case, a minimum temperature of 40° C. and amaximum temperature of 120° C. will be selected. Under these conditions,hydrogenation rates of greater than or equal to 99% can be obtained witha propylene yield that is often more than 100%. However, for feedstocksthat are very concentrated in MAPD (contents greater than 4%) or in thecase of reaching strict specifications (“polymer grade”), thehydrogenation reaction is done with a more or less significant loss ofpropylene.

This last method has been extensively explored in the literature, andhigh-performing combinations have been proposed for the selectivehydrogenation reactions. For example, the U.S. Pat. No. 5,356,851teaches us that it is advantageous to combine a metal from group VIII(preferably Pd) with an element such as indium or gallium for selectivehydrogenation applications of polyunsaturated compounds Likewise, thecombinations Pd—Cu (U.S. Pat. No. 5,464,802), Pd—Ag (U.S. Pat. No.4,547,600), Pd—Sn and Pd—Pb (JP59227829) or else a combination of Pd andan alkaline metal (EP0722776) have been identified for theirhydrogenation performances.

In general, these bimetallic effects are linked to the interactioncreated between the two elements. It thus appears that theidentification of a multimetallic catalytic system is conditioned by theestablishment of this interaction.

Among the methods that make it possible to control the characteristicsof the bimetallic particles (composition, size), it is possible to citethe methods of surface-controlled reactions (US20020045544, J. Barbier,J. M. Dumas, C. Geron, H. Hadrane Appl. Catal 179, 1994, 116 (1-2), S.Szabo, I. Bakos, F. Nagy, T. Mallat, J. Electroanal. Chem. 1989, 263,137) that employ in particular phenomena of surface oxidation-reduction.

This invention describes a catalyst and a method for the preparation ofsubstrate multimetallic catalysts for which the size, the composition aswell as the distribution of the bimetallic particles in the substrateballs are adapted to the requirements of the selective hydrogenationreactions. Actually, it is known to one skilled in the art that for thehydrogenation reactions of polyunsaturated molecules, such as thediolefins or the acetylene molecules, the reaction speed depends on thesize of the metallic particles: this result is generally described underthe term “sensitivity to the structure.” An optimum is generallyobserved for a size on the order of 3 to 4 nm, whereby this value canvary based on, in particular, the molecular weight of the reagents (M.Boudart, W. C. Cheng, J. Catal. 106, 1987, 134, S. Hub, L. Hilaire, R.Touroude, Appl. Catal. 36 1992, 307).

It is thus essential to obtain a particle size distribution(criterion 1) centered on the optimum value as well as a minimumdispersion around this value.

The identification of the bimetallic pairs was the object of numerousworks in the area of selective hydrogenation reactions (V. Ponec, G. C.Bond, Catalysis of Metal and Alloys, Elsevier, Amsterdam, 1995). Thesestudies also revealed the difficulty in obtaining the desired synergyeffect, whereby this effect depends on the method of synthesis selected.Thus, the local composition (criterion 2) of the active phase plays akey role in reaching high catalytic performance levels. Whereby theyields that are obtained are the result of the transformations thatoccur on each particle, the composition of the latter should be adjustedto the optimum formulation and the particles should be homogeneous withone another.

Finally, the macroscopic distribution of elements in the substrate balls(criterion 3) also constitutes an important criterion, primarily withinthe framework of rapid and consecutive reactions such as the selectivehydrogenations. In this case, it is preferable to deposit the elementsin a fine crown at the periphery of the substrate grains so as to avoidthe problems of intragranular material transfer that can lead to a lackof activity and a loss of selectivity.

It is particularly difficult to respect these three criteriasimultaneously, and in general the works that make it possible toproduce such catalysts propose using very sophisticated preparationtechniques that cannot be extrapolated on the scales required forindustrial production (Rousset, J. L., Stievano, L., Cadete SantosAires, F. J., Geantet, C., Renouprez, A. J., Pellarin, M., J. Catal.,197(2) 2001 335).

The methods of impregnation in excess (volume of solution to beimpregnated greater than the pore volume of the substrate mass) orimpregnation in the dry state (volume of solution corresponding to thepore volume of the substrate mass) of acid solutions of metallicprecursors make it possible to vary the size and the overallcomposition. However, on the local scale of the particles, theseparameters often remain very heterogeneous. The use of organometallicprecursors makes it possible to obtain catalysts that are homogeneous incomposition on the scale of the particles, but, in this case, the sizeof the particles is generally small (less than one nanometer), or, forlarger particles, the size distribution around this mean value is broad.In addition, these precursors are used in organic solvents that areunfavorable from an environmental standpoint.

There is therefore described here a new way of preparing bimetalliccatalysts that meet the three above-mentioned criteria.

SUMMARY OF THE INVENTION

The invention relates to a process for the preparation of a catalystthat comprises at least one metal M1 and a promoter M2 on a substrate,whereby said process comprises at least the following stages:

-   -   a) The preparation of a colloidal oxide suspension of a first        metal M1 that consists in the neutralization of a basic solution        by an acidic mineral solution that contains the precursor of the        metal M1,    -   b) Bringing into contact the precursor of the promoter M2,        either directly in its crystallized form or after dissolution in        aqueous phase, with the colloidal suspension that is obtained in        stage a),    -   c) Bringing into contact the colloidal suspension that is        obtained in stage b) with the substrate,    -   d) Drying at a temperature of between 30° C. and 200° C., under        a flow of air.

The invention also relates to a process for the treatment of an olefinicfraction that uses the catalyst that is prepared according to one of thepreceding claims. Said olefinic fraction is preferably a light olefinicfraction that primarily contains hydrocarbons with 3, 4 or 5 carbonatoms.

DETAILED DESCRIPTION

This invention describes a substrate bimetallic catalyst and its processfor production making it possible to prepare catalysts whose particlesare characterized by a small size distribution, a homogeneouscomposition on the scale of the particles, and a macroscopicdistribution in a ring in the substrate balls. A preferred manner ofcarrying out this preparation is as follows: the synthesis comprises thepreparation of the solution containing the bimetallic precursors andthen the deposition on the substrate. A colloidal oxide suspension ofthe first metal M1 is synthesized in the first stage (stage a) bypartial neutralization of a basic solution that contains, for example,an alkaline (soda, potash, . . . ) or ammonia by an acidic mineralsolution that contains the precursor of the metal M1. The promoter M2 isbrought into contact in a second stage (stage b) with the colloidalsuspension of the metal M1 or in dissolved form in an aqueous solutionor directly in crystallized form.

In a third stage (stage c), the solution that contains the two metals isfinally brought into contact with the substrate, preferably in acontrolled manner, for example by means of a spraying system. Thesolution can undergo a maturation stage for a variable period (fromseveral minutes to several hours, preferably 1 or 2 hours) at variabletemperatures (preferably 20° C. to 100° C., more preferably 20° C. to80° C.). In a subsequent stage (stage d), the catalyst is dried under aflow of air so as to evacuate the water that is contained in the pores,then optionally calcined during a stage e under a flow of air attemperatures of between 100 and 600° C., preferably between 150 and 500°C., and even more preferably between 200 and 450° C. The solid isgenerally reduced under a flow of hydrogen before its use in catalysisso as to convert the metallic elements in their reduced form. An in-situreduction (i.e., in the reactor) is possible.

Bringing the impregnation solution into contact with the substrate canbe carried out by impregnation in the dry state or by impregnation inexcess in static or dynamic mode. The addition of stabilizers (anionicsurfactants, cationic or non-ionic surfactants, ions, polymers) canadvantageously be carried out so as to increase the stability of thecolloidal suspensions. For example, compounds such as sulfobetaine,polyacrylate, dodecyl sodium sulfate or else polyethylene glycol can beused without this list being exhaustive. This addition can be madeeither before the first stage of the preparation of the solution orafter the second stage before the impregnation on the substrate.

The metal M1 is selected from among the elements from groups 8, 9 and 10of the new periodic table (CRC Handbook of Chemistry and Physics [ManuelCRC de Chimie et Physique], 81^(st) Edition, 2000-2001, CRC Press,Editor David R. Lide, cover pages), preferably M1 is a noble metal, morepreferably palladium or platinum.

The promoter M2 is selected from among the elements from groups 8, 9,10, 13 and 14. A first list of preferred promoters M2 consists of:nickel, copper, silver, gold, indium, tin and germanium.

Said promoter M2, when it is selected from among the elements fromgroups 8, 9, 10, is preferably different from metal M1. More preferably,the promoter M2 is selected from among the elements from groups 11, 13and 14, more preferably from among the elements from groups 13 and 14,and very preferably the promoter M2 is indium or tin.

The content of metal M1, preferably selected from among the elementsfrom groups 8, 9 or 10 of the new periodic table, is preferably between0.01 and 5% by weight, more preferably between 0.05 and 3% by weight,and even more preferably between 0.1 and 1% by weight. The content ofpromoter element M2 is preferably between 0.5 and 10% by weight,preferably between 0.05 and 5% by weight, and even more preferablybetween 0.1 and 4% by weight.

The substrate of the catalyst according to the invention comprises atleast one refractory oxide that is generally selected from among theoxides of metals from the groups 2, 3, 4, 13 and 14 of the new periodictable, such as, for example, the oxides of magnesium, aluminum, silicon,titanium, zirconium, or thorium, taken by themselves or mixed with oneanother or mixed with other metal oxides of the periodic table. It isalso possible to use carbon. The zeolites or molecular sieves such as X,Y, mordenite, faujasite, ZSM-5, ZSM-4, ZSM-8, etc., as well as themixtures of metal oxides selected from among the metals from the groups2, 3, 4, 13 or 14 with a zeolitic material can also be suitable as wellas the amorphous mesoporous silicic substrates of the MCM or SBA family.The preferred substrate is an alumina and more particularly an aluminawith a specific surface area of between 5 and 200 m²/g, preferably 10 to150 m²/g, and even more advantageously 20 to 140 m²/g.

Before use, the catalyst is generally activated by a treatment under aflow of hydrogen at a temperature of between the ambient temperature andabout 500° C., preferably between 100 and 400° C. This treatment can becarried out in the reactor even where the catalytic reaction will becarried out (in-situ reduction) or previously in independent equipment(off-site or ex-situ reduction).

The process for the treatment of the olefinic fractions according to theinvention comprises bringing said fraction into contact under suitableconditions with the catalyst that is prepared by means of the processfor preparation according to the invention described above. The treatedfeedstocks are light olefinic fractions that contain primarily 3 to 5carbon atoms (fractions that comprise C3, C4 and C5 hydrocarbons) takenindependently after fractionations or in a mixture (C3+C4+C5, C3+C4, orelse C4+C5).

The high selectivities can be obtained by different means: theadjustment of operating conditions (temperature,hydrogen/polyunsaturated compounds ratio), the optimization of thehydrodynamic conditions but primarily by adjusting the catalyticformulation.

The process for the treatment of an olefinic fraction according to theinvention is preferably a selective hydrogenation process. According tothe invention, the treatment of the olefinic fraction is generallycarried out under pressure in a liquid or gas phase, in the presence ofan amount of hydrogen in slight excess relative to the stoichiometricvalue that makes possible the hydrogenation of diolefins and acetylenecompounds. The hydrogen and the feedstock are injected in upward ordownward flow in a fixed-bed reactor whose temperature is between 10° C.and 200° C. The pressure is generally adequate to keep at least 80% byweight of the feedstock to be treated in the liquid phase at the inletof the reactor. It is generally between 0.4 and 5 MPa, moreadvantageously between 1 and 3 MPa. The hourly volumetric flow rate(defined as the ratio of the volumetric flow rate of the hydrocarbonfeedstock to the catalyst volume) established under these conditions isgenerally between 1 and 50 h⁻¹, and preferably between 5 and 40 h⁻¹, andeven more preferably between 10 and 30 h⁻¹.

The process of the invention can also be used according to differenttechnologies, such as, for example, the implantation of the catalyst ina distillation column or in reactor-exchangers.

The following non-limiting examples illustrate the invention.

Example 1 Catalyst A (PdSn, for Comparison)

A catalyst A is prepared by a conventional method of double impregnationin the dry state of acid solutions that contain the molecular precursorsof palladium and tin. 100 cm³ of a solution that contains 0.3 g ofpalladium (example: palladium nitrate) and 0.3 g of tin (example: SnCl₄,5H₂O) is added successively drop by drop to 100 g of alumina substratewith a specific surface area of 120 m²/g, with a pore volume equal to 1cm³/g and with a grain size of between 2.4 and 4 mm, centered on 3 mm.The substrate is dried intermediately at 120° C. between the twoimpregnation stages. After a final drying stage in a flushed bed under aflow of air, the final catalyst contains 0.3% by weight of palladium and0.3% of tin.

Example 2 Catalyst B (Pd—Sn, According to the Invention)

A catalyst B is prepared by bringing into contact a bimetallic colloidalsolution, a solution itself prepared in two stages:

-   -   Partial neutralization of 20 cm³ of a 10N solution of sodium        hydroxide by 20 cm³ of a palladium nitrate solution containing        1.5 g of palladium per liter    -   Addition of 0.28 g of tin, example SnCl₄, 5H₂O crystallized in        the preceding colloidal suspension.

After the volume is adjusted, the solution is then poured into 100 g ofthe same substrate as for Example 1. The catalyst is finally dried at120° C. for 2 hours under a flow of air of 50 l/h. It contains 0.3% byweight of palladium and 0.28% by weight of tin.

Example 3 Catalyst C (Pd—Sn, for Comparison)

A catalyst C is prepared by a first impregnation stage with a porevolume of the substrate of Example 1 by a basic tin solution (exampleSnCl₄, 5H₂O) followed after a drying at 120° C. by a second impregnationwith a pore volume by a palladium nitrite solution Pd (NO₂)₄ ²⁻, itselfprepared by the addition of sodium nitrite to a palladium nitrate acidsolution. The catalyst, finally dried and activated as above, contains0.3% by weight of palladium and 0.3% by weight of tin.

Example 4 Catalyst D (Pd—In, According to the Invention)

A catalyst D is prepared according to the same operating procedure as inExample 2 by replacing the precursor SnCl₄, 5H₂O by InCl₃. The catalyst,finally dried and activated as above, contains 0.3% by weight ofpalladium and 0.3% by weight of indium.

Example 5 Catalyst E (Pd—Sn, for Comparison According to Patent US20010036902)

A catalyst E is prepared by impregnating a colloidal suspension thatcontains palladium and tin. This solution is prepared by adding thefollowing to a soda solution at pH=14: 0.3 g of SnCl₂, 2H₂O and then 20cm³ of an acid solution that contains palladium nitrate initially atpH=0.8. The final catalyst contains 0.3% by weight of tin and 0.3% byweight of palladium. The electron microscopy analysis shows a meanparticle size of 3 nm with a standard deviation of 0.5. The sizedistribution is in accordance with the invention. By contrast, thebimetallicity number is low (50%).

Example 6 Catalyst F (PdNi, According to the Invention)

A catalyst F is prepared by bringing into contact a bimetallic colloidalsolution prepared according to the following stages:

-   -   Partial neutralization of 100 ml of 5N sodium hydroxide solution        by 100 cm³ of nickel nitrate (Ni(NO₃)₂, 6H₂O) at 150 gNi/l).    -   Addition of 0.6 g of palladium in Pd(NO₃)₂ form to the preceding        colloidal suspension.

After adjustment of the volume, the solution is then poured into 200 gof the same substrate as for Example 1. The catalyst is finally dried at120° C. for 2 hours under a flow of air of 50 l/h, and it contains 7.5%by weight of Ni and 0.3% by weight of Pd.

Example 7 Catalyst G (PdAg, According to the Invention)

A catalyst G is prepared by bringing into contact a bimetallic colloidalsolution that is synthesized according to the following stages:

-   -   Dissolution, in 100 cm³ of 5N sodium hydroxide solution, of 45        ml of palladium nitrate at 8.5 gPd/I.

Addition of 0.6 g of silver in the form of silver nitrate to thepreceding colloidal suspension.

After adjustment of the volume, the solution is then poured into 150 gof the same substrate as in Example 1. The catalyst is finally dried at120° C. for 2 hours under a flow of air of 50 l/h, and it contains 0.4%by weight of silver and 0.25% by weight of palladium.

Example 8 Characterization

Table 1 combines the characteristics of the different catalysts in termsof mean size, size distribution through the standard deviation (σ)around this mean value, macroscopic distribution in the substrate, andfinally bimetallicity number. The mean size and the standard deviation(σ) around this mean size

$\left( {\sigma = \sqrt{\frac{\sum\limits_{i}{n\; i\; {Xi}^{2}}}{\sum\limits_{i}{n\; i}} - \left( {\sum\limits_{i}{n\; i\; {Xi}}} \right)^{2}}} \right.$

with ni samples of size Xi) are determined by measurements inTransmission Electron Microscopy (TEM) on a representative particlepopulation. The Castaing microprobe analysis makes it possible todetermine the % by weight of metal present in a substrate ring ofbetween R and 0.9 R, whereby R is the radius of the ball (or theextrudate). The bimetallicity number is also determined by TEM orpreferably STEM (Scanning Transmission Electron Microscopy) equippedwith an x-ray energy dispersion spectrometer also named EDS or EDX(Energy Dispersive Spectroscopy). It corresponds to the percentage ofparticles for which the two metallic elements have been detected with aconfidence criterion (probability) of 95%. This number is calculatedfrom a statistical analysis on a representative population of particles.

TABLE 1 Characteristics of Different Catalysts. % by Weight in a RingEncompassed Mean Size of in R and 0.9 R the Palladium StandardBimetallicity Catalyst Sn Pd Particles (nm) Deviation σ Number % A 40 431.8 2.5 30 B 81 82 1.8 0.4 90 C 22 12 3.3 1.8 85 D 72 69 1.9 0.7 91 E 8085 3 0.6 50 F 70 80 3.5 0.5 95 (Ni) G 86 83 2.1 0.6 92 (Ag)

Example 9 Catalytic Tests on the C3 Steam-Cracking Fraction

A feedstock that comprises 93.43% by weight of propylene, 3.77% byweight of propane, 1.55% by weight of methyl acetylene, and 1.29% byweight of propadiene is treated on the different catalysts presentedabove. Before reaction, the hydrogenation catalysts are activated undera flow of hydrogen at 150° C. for 2 hours. 20 cm³ of each catalyst isplaced in a tubular reactor in upflow mode. The operating conditions areP_(tot)=35 bar, T=25° C., VVH=20 h⁻¹, and the H₂/MAPD ratio variesbetween 1 and 2 mol/mol. The composition of the feedstock and effluentsis followed by gas phase chromatography. The performance levels areexpressed by the ratio of [C₃ ⁼ _(effluent)−C₃ ⁼_(feedstock)]/[MAPD_(effluent)−MAPD_(feedstock)], which represents theselectivity of the catalyst based on the MAPD residual content (MAPD=%by weight of methyl acetylene+% by weight of propadiene).

TABLE 2 [C₃ ⁼ _(effluent) − C₃ ⁼ _(feedstock)]/[MAPD_(effluent) −MAPD_(feedstock)] Hydrogenation Selectivities of a C3 Steam-CrackingFraction for an [MAPD] Residual Content of 10 ppm in the Effluents.Catalyst Selectivity A −10 B 69 C 2 D 58 E 30 F 45 G 55

Example 10 Hydrogenation of a C4 Model Feedstock (Butadiene)

The hydrogenating properties of the different catalysts are evaluatedhere in a “Grignard”-type perfectly-stirred discontinuous test. Twograms of palladium-based catalyst are finely ground (63-100 μm), reducedfor 2 hours at 200° C. under a stream of pure hydrogen, then transferredunder cover gas into the hydrogenation reactor. The feedstock that is tobe hydrogenated is a mixture that contains 12 g of diolefin (butadiene)that is diluted in 180 cm³ of n-heptane.

The temperature of the test is maintained at 20° C. and the pressure at1 MPa. The results are summarized in Table 3. The composition of theeffluents is followed by chromatographic analysis. The hydrogenatingactivity is expressed in mol·min⁻¹·g_(Pd) ⁻¹. The selectivity isexpressed by the ratio of the initial hydrogenation speed of thediolefin to that of the external olefin (butene-1).

TABLE 3 Catalytic Hydrogenation Performance Levels of Butadiene-1,3.Catalyst A B C D E F G Activity 5.7 6.7 4.5 5.1 7.1 6.9 5.2 (mol · min⁻¹· g_(Pd) ⁻¹) Selectivity 1.2 3.6 2.8 3.2 2.3 3.1 3.8

1.-11. (canceled)
 12. A process for the treatment of an olefinicfraction comprising contacting said fraction with catalyst comprising atleast one metal M1 and a promoter M2 on a substrate, said catalyst beingobtained by a process comprising at least: a) preparing a colloidaloxide suspension of a first metal M1 by neutralization of a basicsolution by an acidic mineral solution containing a precursor of themetal M1, b) contacting a precursor of the promoter M2, either directlyin its crystallized form or after dissolution in aqueous phase, with thecolloidal suspension that is obtained in stage a), c) contacting acolloidal suspension obtained in stage b) with the substrate, d) dryingat a temperature of between 30° C. and 200° C., under a flow of air,wherein promoter M2 is an element from groups 11, 13, or 14 of the newperiodic table.
 13. The process according to claim 1, furthercomprising, after drying in (d), (e) calcining under a flow of air attemperatures of between 100 and 600° C.
 14. The process according toclaim 1, wherein the metal M1 is a metal from groups 8, 9 or 10 of thenew periodic table.
 15. The process according to claim 1, wherein themetal M1 is platinum and the precursor M2 is copper, silver, gold,indium, tin or germanium.
 16. The process according to claim 1, whereinthe substrate comprises at least one refractory oxide which is an oxideof the metals from the groups 2, 3, 4, 13 and 14 of the new periodictable.
 17. The process according to claim 1, wherein the substrate is analumina of a specific surface area of between 5 and 200 m²/g.
 18. Theprocess according to claim 1, wherein said olefinic fraction is a lightolefinic fraction that primarily contains hydrocarbons with 3, 4 or 5carbon atoms.
 19. The process according to claim 1, wherein saidcatalyst is used in a fixed bed at a temperature of between 10° C. and200° C., an adequate pressure for maintaining at least 80% by weight ofthe feedstock to be treated in liquid phase at the inlet of the reactor,and an hourly volumetric flow rate of between 1 and 50 h⁻¹.